Advanced silicon bipolar, CMOS or BiCMOS circuits are used today for high-speed applications in the 1-5 GHz frequency range, replacing circuits previously only possible to realize using III-V based technologies.
A common trend in microelectronics is to integrate more and more functions on a single chip, in order to increase the general performance and to reduce size, power consumption and price of the circuits. The versatility of a BiCMOS-process is many time preferred, although it is not suited for all applications. High-performance bipolar integrated circuits have been used extensively for some critical building blocks in telecommunication circuits, mainly for analog functions such as switching currents and voltages, and for high-frequency radio circuit functions such as those in mixers, amplifiers, and detectors. For high-performance cost-effective circuits that would be used in e. g. cellular telephones, a bipolar-only process is many times still to prefer, instead of a BiCMOS process.
For voltage-controlled oscillator (VCO) design, a varactor function, i. e. a voltage-controlled capacitance, is needed for tuning the frequency. It can be realized using the capacitance characteristics of a p/n-junction, which is available in any semiconductor process. In a bipolar process, the base/collector junction would be used, because of the larger capacitance variation, which is set by the doping ratio between the p- and n-side of the junction.
For high-performance VCO design, such as used in cellular systems, the phase-noise of the VCO is an important parameter. It is greatly influenced by the characteristics of the tuning varactor, mostly the Q-value of the varactor (which describes the parasitic losses of device). In M. Steyaert, J. Craninckx, “A fully integrated CMOS DCS-1800 frequency synthesizer”, IEEE J. Solid-State Circuits, Vol. 33, p. 2054, Dec. 1998, the quality of the varactor, which consisted of the p+/n-well junction capacitor in a CMOS process, prevented compliance with the phase-noise specifications over the whole tuning range for a DCS1800 system. Since the diode varactor leaves much to be desired, another way to realize the varactor is needed.
In P. Andreani, S. Mattisson, “On the Use of MOS Varactors in RF VCO's”, IEEE J. Solid-State Circuits, Vol. 35, p. 905, Jun. 2000, different types of MOS varactors are studied for VCO-design. If carefully selecting the device parameters for the varactor, better results than for junction-based varactors are obtained.
Different types of MOS varactors for integration in a conventional CMOS process are described in EP 0902483 A1, as well as many design parameters for practical application of the varactors.
However, for high-performance radio applications where bipolar RF-IC processes are still preferred there is also a need to realize varactors having improved performance.